The present invention relates to feedback controlled or "closed-loop" insulin pumps known also as "artificial pancreases". These devices provide a continuous glucose determination in the diabetic patient. Data is transmitted from a glucose sensor to a microprocessor unit, which controls a pump for insulin, or glucose, infusion, in order to maintain blood glucose levels within a physiological range. Glucose determination in the known closed-loop insulin pumps is achieved by two different methods. In the first, blood is continuously withdrawn from a patient's vein and reaches a glucose sensor, directly or after ultrafiltration. The principal disadvantages of this system are blood loss and/or blood clotting in the drawing tubes. The second method for a continuous glucose assay consists of the insertion of the sensor into the patient's body, usually in the subcutaneous tissue, so that blood is not withdrawn. The drawback of this system is the rapid loss of sensor reliability, probably due to fibrin and some blood cells which unavoidably reach the sensor tip. The aforedescribed disadvantages of the known closed-loop insulin pumps permit only short-term use of the devices.